An example of a conventional method for manufacturing a carbon material for an EDLC electrode is a method in which raw material carbon (pitch) undergoes alkali activation directly or after carbonization (dry distillation).
A problem with the method in which raw material pitch is directly activated is that an EDLC to be provided has electrostatic capacity only on the order of 20 F/cc. On the other hand, it is known that the method in which raw material pitch is activated after dry distillation provides a high level of electrostatic capacity.
For example, Patent Document 1 (Japanese Patent Laid-Open No. 2002-25867) discloses (in the claim 8) a method for manufacturing non-porous carbon containing microcrystalline carbon similar to graphite and having a specific surface area of 270 m2/g or smaller and an interlayer distance d002 of 0.360 to 0.380 for the microcrystalline carbon. The method includes the steps of allowing readily-graphitizing carbon in which multilayer graphite microcrystals have been grown to undergo dry distillation at 700 to 850° C. so as to provide calcined carbon, thermally processing the resultant calcined carbon along with a caustic alkali at 800 to 900° C., and removing residual alkali. An EDLC using the carbon electrode obtained in this method has a high electrostatic capacity of 29 F/cc or greater. Specifically, the EDLC provided in this method uses a carbon material obtained by thermally processing (calcining) petroleum-based needle coke or infusibly-treated pitch as the raw material at 650 to 850° C. in a nitrogen stream for 2 to 4 hours followed by activation.
To manufacture activated carbon for an EDLC electrode by allowing mesophase pitch to undergo an infusibilizing treatment, carbonization and alkali activation, there has been proposed a method in which a massive mesophase pitch undergoes grinding, an infusibilizing treatment, carbonization and alkali activation (Patent Document 2 (Japanese Patent Laid-Open No. 2001-52972)). There have been also proposed a method including the steps of thermally processing (at 400 to 800° C.) raw material pitch having a softening point of 150 to 350° C., an H/C of 0.5 to 0.9, and an optical anisotropic content of 50% or greater, grinding the thermally-processed pitch into particles having an average diameter of 5 to 90 μm, and activating the resultant particles (at 400 to 900° C.) (Patent Document 3 (Japanese Patent Laid-Open No. 2002-93667)). These methods provide a carbon material for an electrode in an electric double layer capacitor having a high electrostatic capacity of 30 F/cc or greater. These methods, however, have a disadvantage that the high electrostatic capacity is not reproducibly obtained and hence cannot develop a high level of electrostatic capacity in a stable manner. To solve this problem, Patent Document 4 (Japanese Patent Laid-Open No. 2004-182504) proposes a method in which pitch having a crystallite thickness in the c-axis direction Lc(002) of 5.0 nm or greater, which is measured by X-ray diffraction, is carbonized and then activated. The raw material pitch having such characteristics is preferably synthetic pitch, which is obtained by polymerizing fused polycyclic hydrocarbon having at least one alkyl substituent group as the raw material at 100 to 400° C. under the presence of hydrogen fluoride and boron trifluoride. It has been also proposed that 5 mass. % of the above synthetic pitch or greater is added to inexpensive pitch having no alkyl substituent group, such as naphthalene pitch and anthracene pitch.
Patent Document 5 (Japanese Patent Laid-Open No. 2003-51430) discloses a method in which raw material carbon containing microcrystalline carbon having a layered crystal structure similar to graphite is heated to 600 to 900° C. and then activated. When the raw material carbon used in this method is a raw material carbon composition in which the interlayer distance d002 determined by X-ray diffraction is 0.343 nm or smaller and the size Lc002 of the crystallites of the microcrystalline carbon determined by X-ray diffraction is 3.0 nm, an EDLC having electrostatic capacity of 30 F/cc or greater is obtained.
Patent Document 1: Japanese Patent Laid-Open No. 2002-25867 (in the claim 8)
Patent Document 2: Japanese Patent Laid-Open No. 2001-52972
Patent Document 3: Japanese Patent Laid-Open No. 2002-93667
Patent Document 4: Japanese Patent Laid-Open No. 2004-182504
Patent Document 5: Japanese Patent Laid-Open No. 2003-51430